Coriolis mass flow meters are well known in the art and are generally described in the following U.S. Pat. No. Re. 31,450, dated Nov. 29, 1983 to Smith and entitled "Method and Structure for Flow Measurement;" U.S. Pat. No. 4,422,338 dated Dec. 27, 1983 to Smith and entitled "Method and Apparatus for Flow Measurement;" and U.S. Pat. No. 4,491,025, dated Jan. 1, 1985 to Smith et. al. and entitled "Parallel Path Coriolis Mass Flow Rate Meter,"; all owned by the present assignee. Such flow meters normally utilize a continuously curved conduit or tube through which the fluid to be measured flows. Oscillation of the tube during flow of the fluid generate a Coriolis force couple which is a function of the mass of the fluid flow rate through the tube.
In order to measure corrosive materials such as HC1 solutions, it is desirable to fabricate the tubing of "Hastelloy" Alloy C, which is a trademark of the Cabot Corporation of Kokomo, Ind. This is a well-known, nickel-based alloy for use in equipment for handling corrosive materials. Such materials have been used in the form of tubing in flow meters and have been torch brazed to stainless steel base members using filler metal alloys of Ag-Cu-Zn. It has been found that the brazements formed by this method have sometimes failed when subjected to tensile stress and the corrosive interaction of the filler metal used in the brazement. In some cases, apparently the thermal stresses resulting from torch brazing have proved sufficient to cause stress corrosion cracking of the tubing by the filler metal.
In metallographic studies, carbide precipitation has been observed in "Hastelloy C-276" samples when the brazing temperatures fell within the range of 1380.degree. F. to 1820.degree. F. (748.degree. C. to 995.degree. C). This is undesirable because it reduces corrosion resistance of the alloy. Apparently, such carbide precipitation occurs at a relatively fast rate. Because of the relatively fast rate of carbide formation, it was found that use of conventionally used brazing filler metals is impractical within this temperature range.
If the "Hastelloy" tubing has a relative large wall thickness, such as greater than about 15 mils (0.38 mm), satisfactory brazements can be obtained using filler metals which contain large proportions of boron, provided the thermal stresses can be eliminated. It has been found, however, that boron tends to diffuse into the "Hastelloy" so that thin walled tubing having a wall thickness less than about 12 mils (0.3 mm) cannot be satisfactorily brazed with a boron containing filler metal.